qtbase fails to detect ICU using UChar=char16_t

ICU auto-detection... ()
x86_64-pc-linux-gnu-g++ -c -march=core2 -O2 -fno-ident -frecord-gcc-switches -pipe -Wall -O2 -Wall -W -fPIC  -I. -I/var/tmp/portage/dev-qt/qtcore-5.5.1/work/qtbase-opensource-src-5.5.1/mkspecs/linux-g++ -o icu.o icu.cpp
In file included from /usr/include/unicode/utypes.h:36:0,
                 from icu.cpp:34:
/usr/include/unicode/umachine.h:306:13: error: 'char16_t' does not name a type
     typedef char16_t UChar;
             ^
In file included from /usr/include/unicode/utypes.h:37:0,
                 from icu.cpp:34:
/usr/include/unicode/uversion.h:165:55: error: 'UChar' does not name a type
 u_versionFromUString(UVersionInfo versionArray, const UChar *versionString);
                                                       ^
...
Makefile:177: recipe for target 'icu.o' failed
gmake: *** [icu.o] Error 1
ICU disabled.
The ICU library support cannot be enabled.